Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus having the same. The present invention is suitably applicable to image pickup optical systems used in, for example, digital still cameras, digital video cameras, TV cameras, and surveillance cameras.
Description of the Related Art
Image pickup optical systems used in image pickup apparatuses are demanded to have excellent optical performance throughout the entire object distance range from infinity to the closest distance. Moreover, they are demanded to be capable of performing focusing at high speed and high accuracy. Especially when performing auto focusing, high focusing speed is demanded.
Image pickup apparatuses such as single lens reflex cameras in nowadays are demanded to have video shooting function and to be capable of auto-focusing during video shooting. As the method of auto-focusing during video shooting, contrast AF (TV-AF) method is widely-employed. In this method, the in-focus state of an image pickup optical system is evaluated by detecting change in the contrast in the picked-up image signal.
In the TV-AF method, an image signal picked up while the focus lens unit is oscillated at high speed along the direction of the optical system is used. In the case where the TV-AF method is employed, it is necessary that the focus lens unit be small and light-weight so that the focus lens unit can be driven at high speed with low load on the driving device (or actuator) that drives the focus lens unit to keep quietness. There have been developed zoom lenses that perform focusing by shifting a lens unit that is relatively small and light-weight among the lens units in them.
Japanese Patent Application Laid-Open No. 2003-295060 discloses a zoom lens that has first to fourth lens units respectively having positive, negative, positive, and positive refractive powers in order from the object side and performs focusing by shifting the second lens unit or the first and second lens units along the direction of the optical axis. Japanese Patent Application Laid-Open No. 2009-251114 discloses a zoom lens that has first to fifth lens units respectively having positive, negative, positive, positive, and positive refractive powers in order from the object side to the image side and performs focusing by shifting the third lens unit.
Japanese Patent Application Laid-Open No. 2012-247687 discloses a zoom lens that has first to fourth lens units respectively having negative, positive, negative, and positive refractive powers in order from the object side and performs focusing by shifting the second lens unit along the direction of the optical axis.
To provide a zoom lens that is small in size, having excellent optical performance throughout the entire object distance range, and capable of performing focusing at high speed, it is important to select the zoom lens type, the number of focus lens units, and conditions in shifting the focus lens units appropriately. In particular, it is important to design the refractive power of the lens units arranged on the object side and the image side of the focus lens units and the configuration of the focus lens units appropriately.
If the design of the above-mentioned factors is inappropriate, it is difficult to provide a zoom lens having a desired zoom ratio and having excellent optical performance throughout the entire object distance range from infinity to the closest distance while achieving size reduction of the zoom lens. For example, if the focusing is performed by shifting a lens unit that is most contributive to the magnification variation of the zoom lens, a large variation of aberration might be caused with focusing. To correct such variation of aberration, it is necessary to constitute the lens unit that is most contributive to the magnification variation by an increased number of lenses. This necessarily leads to an increase in the weight of the focus lens unit, making high-speed focusing difficult.
In the case where focusing is performed by shifting a lens unit that is small and light-weight, it is difficult to achieve excellent optical performance throughout the entire object distance range because of large variation of aberration with focusing, unless the refractive power of the focus lens unit is selected appropriately.